Cyclic Enecarbamates as Precursors of α,β-Unsaturated Iminium Ions: Reactivity and Synthesis of 6,6-Spirocyclic Ring Systems.

The scalable synthesis of cyclic enecarbamates and their use as convenient precursors of alpha,beta-unsaturated N-acyl iminium ions is reported. The newly developed route overcomes synthetic and reactivity difficulties in previously reported methods, is readily scaled up, and proceeds through stable intermediates suitable for long-term storage if required. Preliminary investigations probing the reactivity of cyclic 4-unsaturated N-acyl iminium ions as dienophiles in Diels-Alder reactions and electrophilic alkylating agents are described. In the presence of Lewis and Bronsted acids, iminium precursor 22a underwent efficient Diels-Alder cycloaddition with a range of simple and complex dienes, culminating in the synthesis of 6,6-spirocyclic ring systems possessing the same relative stereochemistry as the spirocyclic imine present in the marine natural product gymnodimine 1.